The present invention relates to differential amplifiers in general, and in particular to differential amplifiers having fast overload recovery.
Monolithic differential amplifiers having unity-gain buffer amplifier input stages are particularly useful in electronic measurement instruments since they not only provide very accurate replication of high-frequency differential signals with high suppression of common-mode voltages but exhibit high input impedance, low noise and low voltage offsets, and operate with relatively low supply voltages and currents.
This type of amplifier, like most amplifiers, typically has a dynamic operating range, or "input window", over which the transfer function is substantially linear. A problem arises in measuring input signals that have voltage swings outside the input window because the amplifier will be driven into a nonlinear region outside the normal operating range, perhaps resulting in an overload that may saturate or cut off a transistor. Also, unbalanced operation due to an overload will result in different power dissipation in the form of heat in some transistors.
Consequently, recovery from the non-linear region following an overdrive condition is delayed not only by the charging or discharging of inherent capacitances, but by thermal changes that continue to distort waveforms until thermal equilibrium is achieved.
One solution to this problem is offered in pending application Ser. No. 06/909,437, filed Sept. 19, 1986, and assigned to the assignee of the present invention. While transistor saturation is prevented and recovery time is substantially reduced, the clamp network added to the basic circuit requires two resistors and an additional transistor on each side of the differential amplifier. It would be desirable to lower the parts count and power supply requirements, and to eliminate heat-producing resistors and the loading that such resistors represent.